On Wednesday, 12 May 2021 at 09:52:52 UTC, Alain De Vos wrote:
As oppposed to what i expect code below prints nothing nothing
on the screen. What is wrong and how to fix it ?
```
import std.stdio;
import std.range:iota;
import std.algorithm:map;
bool mywriteln(int x){
writeln(x);
return true;
}
void main(){
5.iota.map!mywriteln;
}
```
You've got a *lazy* range here. Try `5.iota.map!mywriteln.array`
(you need to import std.array). With that it is not lazy anymore
and will print.
The way, I'm thinking about lazy ranges is in terms of vouchers,
like you sometimes have to buy vouchers on sport festivals to get
sausages. Now you go to the stand with the sausages and hand in
that voucher, you get the sausage. But if you throw the voucher
in a waste paper basket, you'll get no sausages, they are not
even roasted, as long as you do not query them.
What you do above is to buy that voucher and throw it away.
That's why nothing happens.
So in more detail, `5.iota` gives you a voucher (A) for the 5
numbers from 0 to 4. You hand that voucher (A) to `map` together
with a function `mywriteln` and get back a new voucher (B), this
time for applying the numbers to your function. And that voucher
(B) is discarded, because it is never used.
If you would write `5.iota.map!mywriteln.front`, you would say
something like: "Hey `map`, here is my voucher (B), please give
me the first element you promised to me. `map` now takes the
voucher (A) you gave it earlier and does the same, it asks
`iota`: "Hey `iota`, here is my voucher (A), please give me the
first element." And iota hands `0` to `map`, which now calls
`mywriteln(0)`. `mywriteln` now prints the expected `0` and hands
a `true` to `map` and then `map` gives you that `true` (which you
still discard).
Hope, this analogy helps.
